Nanometer-sized MoS2 clusters on graphene flakes for catalytic formic acid decomposition

Article abstract of DOI:10.1021/cs500943b

MoS₂ was deposited on graphene flakes via decomposition of MoS₃ in vacuum at different temperatures (500-800 °C). The materials obtained were tested for catalytic formic acid decomposition, giving mainly hydrogen and carbon dioxide. According to atom-resolved transmission electron microscopy study, a considerable amount of MoS₂ clusters with a mean size of 1 nm was formed on the graphene surface at 500°C. Simulation of the structure of a cluster revealed the presence of Mo-edge atoms. Raising the preparation temperature up to 800°C led to agglomeration of MoS₂ clusters and formation of thin crystalline MoS₂ particles 20-30 nm in size. The sample enriched with the MoS₂ clusters showed 6 times higher catalytic activity at 160°C than the sample with the crystalline MoS₂ particles. This demonstrates that the observed nanometer-sized MoS₂ clusters are responsible for catalysis.

Full text of DOI:10.1021/cs500943b

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Article
Nanometer-Sized MoS2 Clusters on Graphene
Flakes for Catalytic Formic Acid Decomposition
Victor O. Koroteev, Dmitri A. Bulushev, Andrey Chuvilin, Alexander
Vladimirovich Okotrub, and Lyubov Gennadievna Bulusheva
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs500943b • Publication Date (Web): 26 Sep 2014
Downloaded from http://pubs.acs.org on September 27, 2014
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Nanometer-Sized MoS Clusters on Graphene
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Flakes for Catalytic Formic Acid Decomposition
†
Victor O. Koroteev, Dmitri A. Bulushev, * Andrey L. Chuvilin, Alexander V. Okotrub,
,‡
§,
⊥
,
₸,║
†,‡
†
,‡
and Lyubov G. Bulusheva
†
Nikolaev Institute of Inorganic Chemistry, SB RAS, 3 Acad. Lavrentiev ave., 630090
Novosibirsk, Russia
‡
Novosibirsk State University, 2 Pirogov str., 630090 Novosibirsk, Russia
§
Chemical & Environmental Sciences Department, University of Limerick, Limerick, Ireland
⊥Boreskov Institute of Catalysis, SB RAS, 5 Acad. Lavrentiev ave., 630090 Novosibirsk, Russia
₸
CIC nanoGUNE Consolider, E-20018 San Sebastian, Spain
║
IKERBASQUE, Basque Foundation for Science, Bilbao, Spain
*
Correspondence to dmitri.bulushev@ul.ie
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ABSTRACT: MoS was deposited on graphene flakes via decomposition of MoS in vacuum at
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different temperatures (500–800°C). The materials obtained were tested for catalytic formic acid
decomposition giving mainly hydrogen and carbon dioxide. According to atom-resolved
transmission electron microscopy study, a considerable amount of disordered MoS clusters with
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a mean size of 1 nm was formed on the graphene surface at 500°C. Simulation of the structure of
a cluster revealed the presence of edge Mo atoms. Raising the preparation temperature up to
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00°C led to agglomeration of MoS clusters and formation of thin crystalline MoS particles
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with a size of 20-30 nm. The sample enriched with the MoS clusters showed 6 times higher
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catalytic activity at 160°C than the sample with the crystalline MoS particles. This demonstrates
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that the observed nanometer-sized MoS clusters are responsible for catalysis.
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KEYWORDS: MoS clusters; graphene; structure; formic acid decomposition; hydrogen
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production
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. INTRODUCTION
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D materials, layered heterostructures, and low-dimensional hybrids attract increasing attention
over the last few years. Among them is molybdenum disulfide (MoS ), where metal atoms in a
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layer are sandwiched between two sheets of sulfur atoms. MoS monolayers have been isolated
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in a solution after graphene, but became a real fashion after discovery of the scotch-tape
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method for exfoliating of layered materials and the following decision of the Nobel prize
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Committee. At present, a variety of synthetic techniques are employed to form fullerene-like,
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nanotube, needle-like, monolayer, and flower-like MoS structures.
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It is interesting that the structure of the active site in the natural nitrogenase-enzyme for the
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hydrogen evolution reaction (HER) is similar to the structure of the edge site of MoS2.
Additionally, theoretical and experimental studies show that conductivity, magnetism, and
catalytic activity of nanostructured MoS are often provided by edge sites, while basal atoms do
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not contribute substantially to these properties.
However, the edges in flat MoS are not fully
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coordinated and, thus are energetically unstable, which makes formation of closed-shell
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structures thermodynamically preferable.
Moreover, MoS2 may exist in different
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polymorphic forms distinguished by the layer arrangement and electronic properties. Thus,
recent studies showed that as compared to the hexagonal MoS , the trigonal polytype has a
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significantly enhanced electrocatalytic activity in HER, and readily forms intercalates with
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lithium. A high amount of the edge atoms is expected for small-sized clusters, and although
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many research groups are focusing on the development of their synthesis,
formation of
clusters of less than 2 nm is still challenging due to their extremely high surface energy. A
promising way to stabilize the edge MoS atoms could be a growth of nanoparticles on a
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graphitic substrate.
It has been shown that graphene can promote the growth of hexagonal
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MoS layers, although there is a lattice mismatch between these structures.
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In this work, we demonstrate that MoS nanoclusters with an average size of 1 nm can be
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formed on a surface of graphene flakes via annealing of MoS at 500–600°C. Further increase in
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the temperature causes growth of well-crystallized few-layer MoS nanoparticles. Testing of
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MoS /graphene samples in the reaction of formic acid (HCOOH) decomposition demonstrates an
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important role of the 1 nm-sized MoS clusters for catalytic activity. We are not aware of any
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studies of this reaction on MoS -based catalysts performed so far. However, hydrogen
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production from formic acid decomposition was widely studied during last years in the frame of
the green energy concept: this acid can be produced in a high concentration as a by-product of
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hydrolysis of cellulose from biomass, it can be used for hydrogen storage and for catalytic
hydrogenation or hydrodeoxygenation reactions as a hydrogen donor instead of molecular
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hydrogen. Up to now mostly noble metal catalysts are studied for the decomposition while it is
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well known that MoS can participate in the same type of reactions.
A benefit of MoS is that
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molybdenum is cheaper, several orders of magnitude more abundant than noble metals and,
additionally, MoS -based catalysts are tolerant to sulfur impurities that is critical for certain
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applications.
2. EXPERIMENTAL SECTION
Graphene flakes were obtained by thermal exfoliation of graphite intercalated with nitric and
sulfuric acids. The Brunauer-Emmett-Teller surface area and the total pore volume of the flakes
were determined by N adsorption after outgassing of the sample at 200°C for 12 h using an
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Autosorb iQ Station. They were equal to 59 m g and 0.074 cm g , respectively. The obtained
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relatively low surface area is typical for graphene materials prepared by the same method,
which gives a stack of graphene sheets with various thickness. MoS /graphene composites were
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synthesized by impregnation of the graphene flakes with a Mo-containing compound followed
by thermal decomposition as described below. The flakes (100 mg) were suspended in a 20 ml of
water-ethanol (1:1) solution with ammonium thiomolybdate (NH ) MoS (100 mg). The
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thiomolybdate was decomposed by addition of concentrated HCl (0.5 ml), then the solution was
filtered through a membrane and the obtained MoS /graphene sediment was washed by distilled
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water and dried in air. To convert MoS to MoS , the samples were heated in vacuum (10 Pa) at
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00, 600, 700 and 800°C during one hour. The content of Mo in the samples was accepted to be
equal to the surface Mo content (9±3 wt.%) determined by X-ray photoelectron spectroscopy
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(XPS) for 6 samples. A 1 wt.% Pt/C catalyst (Sigma-Aldrich) with a BET surface area of 650 m
-1
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g studied earlier was used for comparison of the catalytic activity.
The structure and composition of the samples were analyzed using high-resolution
transmission electron microscopy (HR TEM), XPS, near-edge X-ray absorption fine structure
(
NEXAFS) and Raman scattering. TEM samples were prepared by ultrasound-assisted
deposition of isopropanol suspension of the material on lacey carbon film grids. The
measurements were done on a Titan 60-300 TEM/STEM microscope (FEI, Netherlands) at an
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acceleration voltage of 80 kV. Simulation of HR TEM images was made by MUSLI code
utilizing atomistic models partially optimized by MM+ potential. NEXAFS and XPS
experiments were performed at the Berliner Elektronenspeicherring für Synchrotronstrahlung
(
BESSY) using radiation from the Russian-German beamline. NEXAFS spectra near the CK-
edge were acquired in the Auger-electron yield mode. The electrons emitted normally to the
sample surface were measured, and the angle between the incident radiation and analyzer was
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5°. The energy calibration was performed relatively to the π* band in graphite (285.4 eV) (full
width at half maximum, FWHM). The energy resolution in the region of the CK edge was 0.25
eV. The overall XPS spectrum as well as C 1s, S 2p, and Mo 3d lines were measured using
monochromatized radiation at 800 eV with energy resolution better than 0.4 eV (FWHM). In the
spectrum analysis, the background signal was subtracted by Shirley’s method. Raman spectra
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were acquired using a Triplemate 1877 spectrometer (Spex, Germany) with a 488 nm Ar laser.
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Catalytic experiments were performed as described earlier.
0.024 g of a MoS /graphene
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sample was placed in a quartz fixed bed reactor of 4 mm internal diameter. All samples were
pretreated in a 2 vol.% H /He mixture at 350°C for 1 h and cooled in He to the reaction
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temperature (100°C). The reaction mixture contained 1.8 vol.% of formic acid in He. All
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experiments were performed with a total flow rate of 51 cm (STP) min . The reactants and
products were analyzed by a gas chromatograph (HP-5890) fitted with a Porapak-Q column and
a thermal conductivity detector.
3. RESULTS AND DISCUSSION
3.1.Electron Microscopy. Low-magnification TEM images show thin graphene flakes
covered by dark nanoparticles with broad size distributions (Fig. 1a,b). These nanoparticles were
attributed to MoS because the electron beam interacts more effectively with molybdenum and
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sulfur than with lighter carbon atoms. Comparing the images of the samples obtained at the
lowest, 500°C (Fig 1a), and the highest, 800°C (Fig 1b), temperatures we conclude that the size
of MoS nanoparticles does not exceed 30 nm in both cases. Most of the nanoparticles
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recognized at this magnification have the size of around 10–20 nm. However, HR TEM analysis
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a 500 C
b 800 C
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o
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d 800 C
Figure 1. Low-magnification (a,b) and high-magnification (c,d) TEM images of the
MoS /graphene samples prepared by annealing at 500°C (a,c) and 800°C (b,d). Some of the
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MoS clusters are shown by circles and triangle in Fig. 1c, and a part of a monolayer nanoisland
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is framed by a square. The arrows point out holes and dislocations in nanoislands. Few-layer
MoS nanoparticles formed on the graphene surface are framed in Fig. 1d.
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Figure 2. HR TEM image of the MoS /graphene sample prepared at 500°C (a) and size
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distribution of MoS clusters (b). Some of MoS nanoislands are framed in Fig. 2a.
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of the material obtained at 500°C reveals hexagonal MoS nanoislands and poorly crystallized
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molybdenum sulfide clusters (Fig. 1c). The nanoislands are mono- and bilayered with an
irregular shape. The presence of holes and dislocations (shown by arrows in Fig. 1c) in the
hexagonal lattice of MoS implies coalescence of smaller crystallites during the synthesis. The
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basal plane size of nanoislands is often under 10 nm, while the clusters consist of several Mo–S
units. With the increase of the synthesis temperature, the amount of small clusters decreases
significantly and monolayer nanoislands transform into thin nanoparticles with a size of 10–20
nm (Fig. 1d, marked with frames). The size distribution of clusters in the MoS /graphene hybrid
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synthesized at 500°C was determined from statistical treatment of the HR TEM image presented
in Fig. 2a. One can see that the graphene surface is densely populated by black spots
corresponding to the MoS clusters, although the MoS nanoislands (some of them are framed in
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Fig. 2a) are also present. A histogram of the Feret diameters shows that the size of the clusters is
mainly in the range from 0.6 to 1.7 nm with the mean value at about 1 nm (Fig. 2b).
In order to recognize the structure of clusters and nanoislands we analyzed the HR TEM
images of the areas framed by triangle and square in Fig. 1c. The enlarged images are presented
in Figs. 3a and 4a, respectively. Although atoms in the cluster are hexagonally arranged, the in-
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plane distance between bright spots is ~2.7 Å (Fig. 3a). This means that some MoS layers are
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enlarged fragment taken from Fig. 1c (shown by triangle). Simulated image for the model of the
MoS cluster (c) with corresponding atomic structure (d – top view, e – side view) where Mo
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atoms and S atoms are grey and yellow, respectively. The scale mark is 5 Å.
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Figure 4. HR TEM (a) and FFT filtered HR TEM (b) images of a fragment of a monolayer MoS2
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where Mo atoms and S atoms are grey and yellow, respectively. The scale mark is 5 Å.
The nanoisland is presented by honeycomb MoS monolayer with a distance of 3.15 Å
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between centers of hexagons (Fig. 4a). According to Bollinger et al. , molybdenum atoms on
ꢁ
the ꢀ1010ꢂ edge (called Mo-edge) without sulfur coverage are thermodynamically unstable and
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the edge reconstructs making Mo atoms fully coordinated with sulfur. The ꢀ1010ꢂ edge (called
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on this edge. Recently, Zhou et al. observed the Mo-terminated edges without sulfur coverage
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in MoS monolayers formed in Mo-rich or S-deficient growth conditions.
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To clarify the atomic positions and exclude underlying graphene lattice, we filtered the images
using fast Fourier transform (FFT). The obtained images of the cluster and monolayer are
presented in Figs. 3b and 4b, respectively. FFT TEM images of three other clusters are shown in
Fig. S1 (Supplementary Information File). They all have a typical distance between the spots of
about 2.7 Å. To make a model of molybdenum suflide cluster, we considered MoS fragments
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laying inclined on the surface of graphene. The HR TEM image of the model is presented in Fig.
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c. The cluster consists of fragments of hexagonal MoS layers, which are inclined relatively to
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the graphene plane at an angle providing a 2.7 Å distance in the top view projection on the
graphene plane (Fig. 3d). A side view projection clearly shows this (Fig. 3e). The composition of
the model is close to MoS and it has a Mo vacancy and some under-coordinated Mo atoms on
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the edges. The fact that small MoS clusters may be not laying parallel to the graphene plane is
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quite interesting itself, as with a decrease of the size the mean surface energy increases and a
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contribution from the cluster alignment could be significant. Recently, Walton et al. have
shown that shapes of MoS nanoparticles and clusters depend on the supporting material. Thus,
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MoS nanoparticles forming on the Au surface are mostly trigonal, while those deposited on
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highly ordered pyrolitic graphite have hexagonal or round shape. In our case, the formation of
MoS starts from clusters, where some layers are not parallel to the substrate, which is not a
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trivial result. To simulate the HR TEM image of the MoS monolayer fragment, we constructed a
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model where the Mo-edges have 100% S-coverage (Figs. 4c,d). Such coverage is due to sulfur
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excess in our synthesis. In case of 50% S-coverage, we would see a shift of the last raw by ½
4
3
period because of the edge reconstruction.
.2. Spectroscopy. It could be expected that Mo carbide is formed on the interface of MoS2
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clusters and graphene surface. It is quite difficult to detect reliably small amount of these species.
NEXAFS CK-edge spectroscopy is considered as the most promising technique for this purpose,
4
6
as Mo carbide provides a sharp intense resonance at 288.5 eV. In the spectra of our samples,
this resonance was absent independently on the temperature of synthesis (Fig. S2). The observed
spectra reflect only the graphene support. In accordance, no formation of Mo C was reported for
2
28,45
different MoS /C systems in the literature.
Therefore, we support the Van der Waals
2
2
8
interaction model for the growth of a MoS cluster on a graphene surface proposed by Shi et al.
2
This is also in agreement with the data that grapheme-based materials interact with MoS only
2
45,47
weakly as compared to oxide supports.
catalytic activity of MoS species.
Such a weak interaction may provide an improved
2
We analyzed the chemical state of molybdenum and sulfur using the sample obtained at 600°C
as an example. The surface concentrations of molybdenum, sulfur, and oxygen determined from
the survey XPS spectrum are 1.5, 2.7, and 4.3 at.%, respectively. The high-resolution XPS S 2p
spectrum was fitted with three 2p –2p spin–orbit doublets separated by ~1.2 eV with the
3
/2
1/2
intensity ratio of 2:1 (Fig. 5a). The 2p3/2 component of the main doublet is located at a binding
2– 48,49
energy of ~162.0 eV characteristic for sulfide ions S .
The 2p3/2 component at 168.6 eV is
6+
2–
50
assigned to the S state realized in SO4 groups. Since after the synthesis the soluble salts
were washed away of the sample, these groups could be located at the edges of MoS2
nanoparticles. A weak doublet with S 2p3/2 energy of 164.2 eV could be assigned to disulfides
2–
51
S2 formed at the MoS edges or elemental sulfur.
2
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The XPS spectrum in the Mo 3d energy region was fitted with eight components (Fig. 5b). The
low-energy peak centered at 226.3 eV and the component at 233.2 eV are assigned to the binding
2–
6+
energies of the 2s electrons of S and S , respectively. The rest part of the components
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originates from three Mo 3d –3d spin–orbit doublets separated by ~3.1 eV with the intensity
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/2
3/2
4
+
Mo
b
a
2-
(MoS2)
S
(
MoS2)
5
+
Mo
6
+
S
6+
2-
Mo
(SO )
4
Figure 5. XPS spectra of S 2p (a) and Mo 3d (b) regions for the MoS /graphene sample prepared
2
at 600°C.
ratio of 3:2. The energy of the doublet increases with increasing degree of Mo oxidation. The
4
+
intense 3d5/2 component at ~229.1 eV corresponds to the Mo state realized in MoS , the
2
5
+
component with the energy around 231.2 eV could be assigned to the Mo state, and the low-
6+
50,52
energy 3d5/2 component at 233.1 eV is due to the Mo contribution.
Notice, that the XPS
analysis was done for the sample after the reduction in a H /He flow at 350°C for 1 h followed
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by catalytic experiments. This treatment removes not only oxygen but also some sulfur
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resulting in the Mo:S ratio slightly lower than 1:2 for our sample. Literature data
for some
MoS -based samples indicate slightly over-stoichiometric ratio of S to Mo even after reduction
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with hydrogen at 400°C. The lower ratio (1.8) in our case is probably related to the presence of
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+
6+
MoS nanoclusters with excess of Mo edge sites. The Mo and Mo species may be formed in
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the sample during storage at ambient conditions. This is in agreement with literature
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4,50,54,55
data.
A relatively high content of such species in our samples indicates the presence of a
high concentration of Mo-edge sites.
Raman spectra of the MoS /graphene samples exhibit three main peaks in the range 300–1800
2
–
1
–1
cm (Fig. 6). The peak located at ~1579 cm corresponds to C–C bond stretching in graphene
Figure 6. Raman spectra of the MoS /graphene samples prepared at different temperatures.
2
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plane (G band). All the spectra have a negligible intensity of the scattering induced by disorder
in graphitic lattice (D band) indicating high perfectness of the substrate. Displacements of atoms
–
1
in the hexagonal MoS layer contribute to the peaks located at ~382 and ~406 cm and assigned
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to the E and A vibration modes.
The peaks are only slightly shifted with increasing the
2
g
1g
temperature of the MoS /graphene synthesis, while their broadness progressively decreases. This
2
behavior is due to the increase of MoS particle sizes and crystallinity with the rise of the
2
synthesis temperature from 500 to 800°C. Comparing the spectra normalized to the intensity of
graphitic G band, one can see that the amount of crystalline MoS for the samples obtained at
2
5
00 and 600°C is nearly the same and the further temperature rise results in the observed
increase of the MoS peaks intensity provided by the increasing amount of MoS crystal
2
2
nanoparticles. These data are in accordance with the TEM observations (Fig. 1c,d) demonstrating
disappearance of the MoS clusters and growth of bigger hexagonal particles with temperature.
2
3
.3. Catalysis. The MoS /graphene samples obtained at different temperatures were studied in
2
catalytic decomposition of formic acid vapor (Fig. 7). The formic acid decomposition is seen at
about 100°C for the samples synthesized at 500 and 600°C and reaches 100% conversion at
about 250°C (Fig. 7a). With an increase of the synthesis temperature from 500 to 800°C the
conversion curve shifts by about 30°C to higher temperatures and the reaction rate decreases 6
times at 160°C (Fig. 7b). Activation energies were calculated for the formic acid decomposition
-1
from the Arrhenius plots (Fig. S3). They were equal to 81±4 kJ mol being independent on the
synthesis temperature. This clearly indicates that the observed difference in the catalytic activity
is related to a change of the concentration of active sites, but not to a change of the activation
energy. This result is consistent with the Raman scattering measurements (Fig. 6) showing
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.0
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o
Synthesis temperature, C
Temperature, °C
Figure 7. Steady-state conversions of formic acid versus temperature for the MoS /graphene
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catalysts prepared at different temperatures (a) and rates of the formic acid decomposition at
1
60°C as a function of the synthesis temperature (b).
1
.0
.8
CO2
0
0.6
H2
0
0
0
.4
.2
.0
HCOOCH3
CH OH
3
CO
CH4
320
1
20
160
200
240
280
Temperature,
°
C
Figure 8. Selectivity to different products at different temperatures for the MoS /graphene
2
catalyst prepared at 500°C.
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aggregation of MoS with temperature and indicates that the MoS clusters of about 1 nm size
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observed by TEM (Fig. 2b, 3) are the major active catalytic species.
The products of formic acid decomposition are mostly composed by CO , hydrogen and
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methyl formate (Fig. 8). Water vapor was not controlled in our study, but it must be formed
according to a hydrogen balance. CO and hydrogen are typical products of formic acid
2
3
2,39,41,42
dehydrogenation on noble metal catalysts.
It is seen that the hydrogen selectivity
increases with temperature/conversion while the selectivity to CO almost does not change. The
2
formation of methyl formate indicates that hydrogen produced from formic acid may reduce this
acid to methanol or methoxy species, which can further interact with formic acid giving the ester.
Consequently, at higher temperatures the traces of methanol and methane are observed. It is also
seen that the selectivity curve of the methyl formate formation passes through a maximum as a
function of temperature. At high temperatures, this ester can be hydrolyzed by water vapor
present in the products giving methanol and formic acid. This was confirmed by experiments
with addition of water vapor to the reaction mixture containing formic acid (Table 1). The
addition led to a decrease of the methyl formate selectivity and an increase of the methanol
selectivity. The selectivities to the other products as well as the conversion did not change
noticeably in the presence of water. When the decomposition temperature reaches 200°C, traces
of CO are observed. This product can be formed as a result of the dehydration of formic acid or
reverse water-gas shift reaction. We have not observed any reaction of CO with hydrogen (0.5
vol.% CO, 1.6 vol.% H in He) on the studied samples at temperatures up to 450°C. Thus, neither
2
of the products observed is a result of hydrogenation of CO.
5
8
Recently, Koos and Solymosi studied formic acid decomposition over Mo C catalysts
2
supported on different forms of carbon at conditions similar to ours. As they worked at the same
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Table 1. Effect of Water Vapor (2.5 vol.%) on the Conversion and Selectivity of Products
Formation for the Formic Acid Decomposition at 235°C on the MoS /graphene Catalyst
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Prepared at 500°C.
Conversion
Selectivity
CO
Methyl
formate
H2
CO2
Methanol
CH4
Without H O
0.920
0.863
0.42
0.49
0.83
0.89
0.026
0.020
0.121
0.048
0.017
0.043
0.003
0.002
2
With H O
2
range of temperatures, the activities of carbides and sulfides are close. This probably indicates to
similar active sites for the decomposition reaction. Koos and Solymosi supposed that carbon
deficient sites of Mo C could be important for the formic acid decomposition taking place via the
2
formation/decomposition of formate species. The formate route was confirmed later by density
5
9
functional theory calculations of Luo et al. These considerations could be important also for
MoS catalysts. As compared to the MoS catalysts, the Mo C catalysts gave higher hydrogen
2
2
2
selectivity (95–100%) and provided no formation of methyl formate and methanol. However, it
should be taken into account that further improvement of the properties of the MoS catalysts by
2
doping seems possible and using of Mo C as a catalyst is complicated as this material is highly
2
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We compared also the activity related to the content of Mo for the best MoS /graphene catalyst
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with the activity related to the content of Pt for the Pt catalyst supported on activated carbon at
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9
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45°C (Table S1). The mean size of Pt clusters was equal to 1.6 nm, which was quite similar to
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the size of MoS clusters. The activity of Pt was 7 times higher. However, it should be taken into
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account that the price of Mo is considerably lower than that of Pt.
4. CONCLUSIONS
We have shown a principal possibility to synthesize nanometer-sized MoS clusters on the
2
graphene surface. Decreasing of the decomposition temperature in the synthesis to 500°C has led
to high concentration of disordered one nm-sized MoS clusters in the material. At higher
2
temperatures the aggregation of the clusters into well crystalline islands of 20–30 nm size has
been observed. The increase in the size of MoS particles and their crystallinity correlate with the
2
decrease of the catalytic activity of the MoS /graphene material in formic acid decomposition.
2
This correlation allows suggesting that this reaction occurs on the edge Mo atoms exposed
mainly by the MoS clusters. Optimization of the synthetic procedure aiming to a MoS /graphene
2
2
catalyst containing solely 1 nm - sized clusters may lead to highly active catalysts for different
important reactions taking place on MoS edge sites like hydrogen evolution reaction or
2
hydroprocessing of petroleum fractions. It is also important that formic acid could be used not
only as a probe to test active sites of MoS catalysts, but also as a hydrogen donor for reactions
2
involving hydrogen and these catalysts.
AUTHOR INFORMATION
Corresponding Author
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*
E-mail: dmitri.bulushev@ul.ie .
Notes
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The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information
Comparison of the catalytic activity of MoS /graphene and Pt/C catalysts (Table S1), HR TEM
2
images of MoS clusters (Fig. S1), CK-edge NEXAFS spectra (Fig. S2) and Arrhenius plots (Fig.
2
S3). This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENTS
The synthetic part of the work was done with the financial support of the Russian Foundation for
Basic Research grant 14-03-31633. This publication has also emanated from research conducted
with the financial support of Science Foundation Ireland under Grant Number 06/CP/E007.
Collaboration between partner institutions was partly supported by FP7-PEOPLE-2011-IRSES
N295180 (MagNonMag) grant.
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